1. Field of the Invention
The present invention relates to a random pulse type radar apparatus for use in vessels, airplanes, automobiles, missiles and so forth.
2. Description of the Related Art
A conventional typical pulse radar apparatus mainly comprises a transmitter which transmits electromagnetic pulse wave energy at given time intervals, and a receiving unit which continuously monitors echo energy of the transmitted energy.
Because the monitorable distance range of this system is a half the distance acquired by dividing the propagation speed of radio waves by the time interval of output pulses, this system cannot detect echo signals from a target which is located at a greater distance than this monitorable range.
This necessitates that the intensity of the output radio waves should be powerful enough to be dangerous to human bodies, such as radiation at a very close range. In this respect, radar usage has been managed through the license system.
A CW radar, which continuously sends out transmission radio waves and utilizes interference of the radio waves with their echoes from a target, has been put to a practical use. While the CW radar is mainly used to measure the moving speed of a target, the target position can be measured by additionally using frequency wobble. The CW radar is hardly used except for a special application.
A pulse compression radar apparatus of the type that sends out a chirped pulse signal, which is one type of spectrum spread signal, or a BPSK signal of a digital code like a barker code, as an output signal in a short period of time is also used for a special application. This radar apparatus does not differ essentially from the typical pulse radar, and suffers a limited effect of reducing the peak power. In a case of using a BPSK signal, for example, if one tries to reduce the peak power by increasing the transmission time, physical measurement of echoes from a target at a very close range is not possible due to the short reciprocation time of radio waves (see FIGS. 6 and 7).
As pleasure-boats become popular, a radar apparatus which can be used without any danger and special knowledge is strongly demanded from a viewpoint of preventing sea accidents. Such a radar apparatus is not actually available on the market because there are no adequate technical schemes.
It is known that the precision of measuring the distance by a radar becomes higher in proportion to the band width of radio waves that are generally used.
In the field of radio communication, the direct sequence (DS) system which employs PSK (Phase Shift Keying) modulation by using recursive code sequences and the frequency hopping (FH) system which switches transmission frequencies at a high speed have already been used widely as schemes of generating wide-band radio waves.
When radio waves of those systems are adapted for a radar usage, the peak of the transmission radio waves becomes minimum but the radio waves are output continuously. In a case of a radar apparatus in which its antenna is used as both the transmission antenna and reception antenna, however, masking of radio waves by the transmission signals makes it substantially impossible to receive very weak echoes.
Accordingly, it is an object of the present invention to significantly reduce transmission peak power by employing a system, which is disclosed in Japanese Patent No. 2655374, entitled xe2x80x9cSpectrum Spread Communication Equipmentxe2x80x9d, by the same applicant as that of this invention and which sends out as an output signal a spectrum spread radio wave including a xe2x80x9csignal-less periodxe2x80x9d set at a pseudo random and receives echoes in this xe2x80x9csignal-less periodxe2x80x9d.
To achieve the object, a random pulse type radar apparatus according to this invention comprises a transmitter for generating a hybrid type spectrum spread signal by simultaneously using two kinds of modulations which are phase shift keying (PSK) modulation for selecting a phase of a transmission radio wave in accordance with a pseudo noise digital code and outputting the transmission wave, and time hopping modulation for stopping transmission of a radio wave at random in accordance with the pseudo noise digital code; a receiving unit for selectively detecting an echo of a transmission signal radio wave, generated by the transmitter, from a target with a time delay; at least one a common antenna unit for use both for transmission and reception or antenna units installed close to each other and respectively serving single functional units; and a reception control unit for stopping an action of the receiving unit in a time zone in which the transmitter is outputting radio waves in accordance with the time hopping modulation, whereby a spatial distribution of an intensity of an echo of a transmitted radio wave is measured through computation of a cross-correlation function of a transmission signal and a reception signal.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.